Dipyrromethene and β-Diketiminate Zinc Hydride Complexes: Resemblances and Differences

Abstract

A new dipyrromethene (DPM) ligand with bulky DIPP-substituents is introduced (DIPP = 2,6diisopropylphenyl). The ligand, abbreviated as DIPP DPM, was deprotonated with ZnEt 2 to give ( DIPP DPM)ZnEt, which reacted with I 2 to form ( DIPP DPM)ZnI. Reaction of the latter with K[N( i Pr)HBH 3 ] afforded a labile Zn amidoborane complex which, after β-hydride elimination, formed ( DIPP DPM)ZnH. Crystal structures of ( DIPP DPM)ZnX (X = Et, I, H) revealed their monomeric nature. The Zn−N bond distances are somewhat longer than those in the corresponding monomeric β-diketiminate complexes ( DIPP BDI)ZnX ( DIPP BDI = CH[C(Me)N-DIPP] 2 ). This is in agreement with calculated NPA charges, which are lower on the N atoms of DPM compared to those on BDI. Reaction of ( DIPP DPM)ZnH with CO 2 gave ( DIPP DPM)Zn(O 2 CH), which crystallized as a monomer with a symmetrically bound η 2 -formate ligand. In contrast, the β-diketiminate complex crystallizes as a dimer [( DIPP BDI)Zn(O 2 CH)] 2 with bridging formate ligands. Reaction of ( DIPP DPM)Zn(O 2 CH) with various silanes regenerated the hydride complex ( DIPP DPM)ZnH. Catalytic CO 2 hydrosilylation with (EtO) 3 SiH using ( DIPP DPM)ZnH as a catalyst gave full reduction to [Si]−OMe species, whereas the catalyst ( DIPP BDI)ZnH only partially reduced CO 2 to [Si]−OC(O)H. The advantage of the DIPP DPM ligand is the arrangement of the DIPP-substituents, which form a pocket around the Zn−X unit, preventing dimerization and influencing its reactivity. In addition, in contrast to the negatively charged central backbone carbon in the DIPP BDI ligand, that in DIPP DPM is neutral. This makes it less nucleophilic and Brønsted basic, as expected for a true spectator ligand.

Stabilization of Heteroleptic Heavier Alkaline Earth Metal Complexes with an Encapsulating Dipyrromethene Ligand

“…Compared to the same series of ( DIPP BDI)AeN′′ complexes (Ae = Mg, Ca, Sr, Ba), the Ae‐N distances in the DPM complexes are consistently 0.01–0.08 Å longer. This fits with our earlier observations for DIPP DPM zinc complexes for which it could be shown that the DPM ligand is clearly weaker bound to the metal than the corresponding BDI ligand . DFT calculations support this observation and show that, compared to BDI complexes, the negative charge on the N atoms in DPM ligands is lower.…”

“…DFT calculations support this observation and show that, compared to BDI complexes, the negative charge on the N atoms in DPM ligands is lower. The latter can be explained by the large number of mesomeric structures for the DPM anion, indicating efficient delocalization of negative charge over the appended five‐membered rings …”

Backbone Reactivity of Lithium β‐Diketiminate (NacNac) Complexes with CO2, tBuNCO and iPrNCO

“…[27] Harder has recently unveiled an ew variation of the DIPPBDI ligand whichh as been termedD IPPDPM, based upon ar elatedd ipyrromethene scaffold. [28] In contrastt ot he negatively charged central backbone carbon found in DIPPBDI offering nucleophilic reactivity,t hat in DIPPDPM is effectively neutral. Thism akes it al ess nucleophilic and less Brønsted basic molecule, as appropriate for at rue spectatorligand.…”

“…In contrast to the negatively charged central backbone carbon found in DIPPBDI offering nucleophilic reactivity, that in DIPPDPM is effectively neutral. This makes it a less nucleophilic and less Brønsted basic molecule, as appropriate for a true spectator ligand …”

“…Thism akes it al ess nucleophilic and less Brønsted basic molecule, as appropriate for at rue spectatorligand. [28] Next we extended our studies to another class of small molecule,n amely isocyanates. This seemed ap ertinentc hoice given recent work by Ma et al that explored the activation of small molecules, including isothiocyanates, by am agnesium alkyl NacNac complex.…”